4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

MECHANISM AND DYNAMICS OF THE ROUGHENING OF PAPER
IN CONTACT WITH MOISTURE

Toshiharu Enomae and Pierre Lepoutre

Paper Surface Science Program, University of Maine, USA

ABSTRACT
When paper is contacted by water or simply exposed to high humidity, the fibers swell and bonds break.
The paper structure is altered; it loses strength and it roughens, adversely affected the print quality. Several
hypotheses have been proposed to explain this roughening. Among them: Fibers swell and this causes a
shape reversion from ribbon to tube; fibers swell and this leads to bond breakage; drying stresses arerelaxed by the water or high humidity, and the paper structure changes, reaching a new equilibrium
structure; the calendering stresses, that keep mechanical fibers flattened, are relaxed by water and the fibers
return to their tubular shape.

There is now experimental evidence to support the view that surface structural changes are the result ofchanges both at the surface and in the bulk. Kinetics of the roughening phenomenon have been measured
using a special device that monitors simultaneously the change in gloss (a good measure of roughening)
and in moisture content when paper is subjected to a step change in relative humidity. Results show thatgloss lags behind moisture pickup, confirming that bulk structural changes contribute largely to the surfacechanges.

It has been shown that it is not the changes induced by water, per se, that are the problem, but theirirreversibility in a cycle of wetting and drying. Indeed some papers exhibit strong roughening in theenvironmental microscope but return to their smooth original state after drying.
The phenomenon of shape recovery (from ribbon to tube) has been given particular attention. Forseth et al1)
have examined the decollapse of mechanical fibers in calendered papers, using image analysis on
cross-sections. Their work shows a systematic correlation between amount of fiber decollapse androughening, the effect increasing with cell wall thickness. The work also shows the effect of decollapse in
the bulk of the sheet. We have examined the shape recovery from ribbon to tube for the case of Kraft fibers.
Kraft fibers in a handsheet were examined after soaking in water, after defibering, after beating andcompared to fibers from a reslushed lap pulp and the never-dried pulp by examining cross-sections offreeze-dried specimens. In the handsheets, surface fibers showed more extensive swelling than those inside,
presumably because they are constrained from one side only. The collapsed fibers in the handsheet returnedto the tube-like shape of the never-dried pulp, but only after beating.
1

4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

It is concluded that, only in the case of mechanical fibers, is shape recovery a significant contribution to theroughening process. In mixed furnishes, swelling of the Kraft fibers may assist in decollapsing mechanicalfibers.

INTRODUCTION
Water is applied to paper during coating and during

What’s the Problem ?

printing, such as offset, water-based gravure, flexo• Water is applied during:– coating– printing (offset, water/based gravure, flexo, inkjet)• It interacts with fibers and fiber network– It causes dimensional changes
in-plane (cockling, misregister)and out-of-plane (roughening, fiber-rising)that affect print quality

and inkjet. It interacts with fibers and fiber network.
It causes dimensional changes in plane as
exemplified by cockling that occurs because of
non-uniform drying and shrinking and misregisterbecause of shrinking and out-of-plane as
exemplified by surface roughening and fiber risingthat all affect print quality.

The effectNow, practical experiences of surface roughening
are taken into account. In coating, coating• Coating.
– Coating smoothens surface by filling up voids but– Water roughens up surface

smoothens surface by filling up voids but water ina coating color roughens up surface. In printing,• Printing

gloss drops and becomes non-uniform. Fibrous– Gloss drops and becomes non-uniform– Fibrous texture appears behind the coating:“fiber-rising”

texture appears behind the coating. This is called“fiber-rising.”

These three electron micrographs show how fiber
rising happens. A specimen of
uncalendered Light-Weight-CoatedFiber rising in ESEM

paper was observed in Environmental
Scanning Electron Microscope,
C ooledb elow d ew
poin twhich permits to observe samples inhumid environment2). When water
D ry

condensed on the coated paper
surface (see picture "Wet"), fibers

W et

R etu rne d toroom te mp

appeared to rise and push up thew atercoating layer. The surface was roughin the wet state. Then, when theR e d ry

sample is dried again (see picture
2

U n c a le n d e r e d L W C

4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)"Redry"), the fibers returned to the

Sample chamber of GlossMachineoriginal locations and the surface
looked similar to the initial dry one

Air out(see picture "Dry").
Reference
Light source

To acquire a roughening
Air in atcertain RH

phenomenon quantitatively, a novel

Detector
Air in atcertain RH

Paper

apparatus was developed to measureSample stage

gloss changes. The figure is a sketchof the GlossMachine3). A paper

Shaft connectedwith balancesample is attached to a metal stage
with two-sided adhesive tape. The
stage is directly supported on a top-loading balanceso that sample weight can be converted into

Kinetics

moisture content. The chamber ceiling is equippedwith a light source and detector for gloss• GlossMachine measures simultaneously:– Gloss change– Moisture pickup
during step change in humidity

measurement set at equidistant angles of 75
degrees. A metal plate is used as a reference. The
ratio of paper gloss to that of the reference plate
provides a relative gloss value. This system is forcompensating for electrical drift and for
eliminating absorption of light in the near infrared

band by water vapor. Humid air or dry air is supplied through small openings.

GlossMachine measures simultaneously gloss change and moisture pickup during step changes in humidity.

RESULTS AND DISCUSSION
These figures show changes in gloss

Gloss kinetics

110

Switching from0 to 90 % RH

100

15

10
90

and moisture content for580

the sample was moistened at 90%
relative humidity. On switching to
90% RH, the moisture content
quickly increased and continued toincrease even after 12 hours. Gloss

60
70

100

0
-1 0

0

50
10

80

40

60

30

40

20

20

10

0

0
0

2

4

6

8

10

12

T im e, hour

decreased correspondingly, but its
3

14

16

18

20

30

40

50

60

70

80

90

T im e , m in

M oisture content, %wood-containing paper. After drying,

120

GlossM achine-Gloss, %supercalendered uncoatedSupercalendered,
uncoatedwood-containing paper4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

initial rate of decrease was lower than the rate of moisture pickup as shown by the top right blow-up. The
fact that gloss lags behind moisture pickup and is still decreasing well after the equilibrium at surface
suggests that the surface continues to reflect the reconfiguration of fibers across the bulk even after a longtime. In this sense, gloss relaxation can be considered, paradoxically, as a "bulk" process.

The figure shows the gloss relaxation

0 - 90 % RH cycles

cyclically. Moisture content changed
the same way every cycle. However,
the gloss dropped step-by step during
the humidity cycles. This way, gloss

GlossMachine-Gloss, %1 20

relative humidity was varied

1st cycle
90 %

1 00

60

2nd cycle

0%

90 %

0%

50

80

40

60

30

40

20

20

10

reduction was accompanied by0irreversible changes as if mechanical

Moisture co ntent, %and moisture content change as

0
0

2

4

6

8

10

12

14

16

18

20

T im e, h o u r

stress relaxation had occurred.

Surface mechanisms
R oughening
PossibleofMechanisms
roughening• Gloss-change lags behind moisture pickup
in the bulk.
• Thickness and roughness changes go
hand in hand.

Fibe rsw ellin g

Conclusion:

Recov ery to un co llapse dorigin al sh ap e

C oatin g• Roughening is not only a “surface effect”.
But the reflection of irreversible structuralchanges in the bulk.

S tre ssr elax a tio n

D ebondin g

R es po nse of pap er surfac e to wate r in co at in g
[J . Sk o wron s ki, J P P S 16 , N o .3 (5), J 1 02 (1 99 0 )]This (above left) summarizes findings regarding gloss kinetics. Gloss-change lags behind moisture pickup
in the bulk. Thickness and roughness changes go hand in hand. So, the conclusion here is: Roughening is
not only a “surface effect”, but the reflection of irreversible structural changes in the bulk.

This schematic diagram (above right) illustrates what kind of structural changes happens with a surface anda bulk of paper in contact with moisture or water. Possible mechanisms of surface roughening are fiberswelling, stress relaxation, fiber-fiber debonding and recovery to uncollapsed original shape. In addition,
irreversibility is combined.

Details of the possible mechanisms indicated by the diagram will be discussed. The first one is relaxationof internal stresses. Internal stresses can be created during drying or during calendering. Internal stresses
44th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

are relaxed by the plasticization effect of water.

Mechanisms

Elements under stress, whether microfibrils or fiber• Relaxation of internal stresses

segments, are now free to move to relax stresses.

– Internal stresses can be created during drying or during
calendering.

The relaxation is evidenced by: In-plane and– Internal stresses are relaxed by the plasticization effect ofwater. Elements under stress (whether microfibrils or fiber
segments) are now free to move to relax stresses.
• Evidence:out-of-plane movements of surface fibers in the
ESEM, and shape recovery: Fibers which were not– In-plane and out-of-plane movements of surface fibers in
the ESEM.
– Shape recovery: Fibers which were not collapsed before
calendering tend to recover their uncollapsed shapecollapsed before calendering tend to recover theiruncollapsed shape. Both are observed mostly with

Observed mostly with stiff, thick-walled mechanical pulp fibers

stiff, thick-walled mechanical pulp fibers.

The second possible mechanism is debonding.

Mechanisms (cont’d)

Cross-sectional changes due to swelling or shaperecovery push apart bonded fibers when the bonds• Debonding– cross-sectional changes due to swelling or shape recovery
push apart bonded fibers when the bonds are weak andfurther weakened by water– If bonds break, fiber elements under stress may moveirreversibly– Evidence ?

are weak and further weakened by water. If bonds
break, fiber elements under stress may moveirreversibly. This is evidenced by that in-plane• In-plane drop in tensile strength is about 10% after wetting andredrying(Forseth, 1996)• But effect is larger at surface since one-side bonded

drop in tensile strength is about 10% after wetting
and redrying, but the effect is larger at surface
since one-side bonded.

Mechanisms (cont’d)Mechanisms (cont’d)• Fiber swelling• Fiber swelling– BK fibers have the highest swelling potential(WRV increases by 100% as a result of beating)– Swelling stresses are significant (Skowronski,
1988)– Swelling is quite “visible” in the ESEM– One would expect swelling stresses to cause
collapsed kraft fibers to go from ribbon to tube ?

– Measurements on BK beech fiber cross-sections(round, thick-walled) reveal expansion about 710% between 10 and 90% RH (Nanko & Tada,
1995)
Presumably much larger with liquid water.

The third possible mechanism is fiber swelling (above left): There are a few evidences reported. Bleachedkraft fibers have the highest swelling potential considering that Water Retention Value increases by 100%
as a result of beating. Swelling stresses are significant. Swelling is quite visible in the ESEM. One would
expect swelling stresses to cause collapsed kraft fibers to go from ribbon to tube?

And (above right), measurements on bleached kraft beech fiber cross-sections that are round andthick-walled reveal expansion about 7-10% between 10 and 90% relative humidity. Presumably, it would
be much larger with liquid water.
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4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

Fiber shape changes
The last possible mechanism for surface• Mechanical • Chemical
pulp
pulp

roughening is fiber shape changes. MechanicalPulping

pulp fibers are collapsed during calendering in the

Wet-pressing

papermaking processes. The fibers recover a

DryingCalenderingtube-like shape on re-wetting. In contrast, chemical

Re-wetting

fibers are easily collapsed during wet-pressing and?

Re-drying

drying and even more during calendering. Afterre-wetting and re-drying, fibers are collapsed. But,
what shape do chemical fibers takeon re-wetting? This was the objectiveof our next experiment.

Fiber Shape recovery[Forseth,
NPPRJ
12(1):67-71(1997)]
or set.
et h,al.,T. et.
al., NP PR
J 1 2 (1 ):67 -7 1)(F T.

M echanical pulp
❚ B efore

Before explaining the experiment,

supe rcalend ering

here is shape recovery behavior ofmechanical fibers already reported.

❚ S uper calende red

Mechanical fibers collapsed during
supercalendering recovers a tube-like❚ A fter so aking inshape greatly after soaking in water

w ater for 2 s

and fairly after exposure to high

❚ A fter 9 7 % R H
for 24 hhumidity.

Samples - softwood kraft

Objective

to observe in the ESEM
Pulping• To determine to what extent
kraft fibers, isolated, or in paper,
recover a tube-like shape oncewetted with water

Never-dried pulp

Freeze-drying

Sample 1

Air-drying
Lap pulp

Defibering

Beating

Handsheet

Sample 2

SoakingFreeze-drying

Sample 3

Defibering

Freeze-drying

Sample 4

So, the objective (above left) of our next experiment is to determine to what extent kraft fibers, isolated, orin paper, recover a tube-like shape once wetted with water4).

Samples were prepared following this flow-sheet (above right). Sample 1 is freeze-dried fibers preparedfrom never-dried kraft pulp. Sample 2 is a handsheet made from dry lap pulp after defibering and beating
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4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

without any additives. After ESEM observation of Sample 2, the specimen was soaked in water andfreeze-dried. This is Sample 3. Sample 4 is freeze-dried fibers prepared from soaked and defibrated
handsheets.

Never-dried pulp

Fibers from defibrated handsheet
Lap pulp

Never-dried
pulp

Defibering

Freeze-drying

Soaking
Beating
Handsheet
Soaking
Defibering
Freeze-drying

The micrograph (above left) shows
fibers of never-dried wet pulp

Handsheet Surface

directly freeze-dried. Never-dried
fibers appear to be tube-like and not

Surface of handsheet made from beaten softwood kraft pulp

to be collapsed.

The micrograph (above right) shows
freeze-dried fibers prepared fromsoaked and defibrated handsheets.
Fibers look all collapsed. Defibering

Dry sheetshould give a modest but certain

After soaking and freeze-drying(Wet sheet)effect on fiber swelling, so someswelling effect should be included in
the shape. However, apparently theeffect was not great enough to giveHandsheet Surface

rise to recovery to tube-like shape.

( Edge- ext ract ed)
Surface of handsheet made from beaten softwood kraft pulp

To examine fiber shape recovery

1

1

C

2

more precisely, the same fibers on a3

handhseet surface were followed.
The two micrographs show a surface

B

4

2

A

1
2

of a dry handsheet and the same

Dry sheetsurface after soaking and7

After soaking and freeze-drying(Wet sheet)4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

freeze-drying, which reproduces the wet state. They are very similar. However, it was found that somefibers shrank laterally very slightly by wetting.

If fiber edges are emphasized by image analysis, fiber width change is easily measured. The fiber width
reduction was measured along indicated lines of the three fibers A, B and C.

The table shows the data of measured
width shrinkage, which are 4.0, 9.0

Estimated Shape Change

and 11.5 %, respectively. If the fiber
wall is assumed not to swell, the fiber
shape can be predicted as shown by

Thickness
Widthr, µm a, µm b, µm
increase, %shrinkage, %the sketches in the last column.

0

3

11

0

0

4.0

3

10.45

0.55

20

Fibers A and B had a potential to(Fiber C)

recover some degree of tube-like(Fiber B)

9.0

3

9.75

1.25

40

shape. Probably, it is because those

11.5

3

9.40

1.60

50

(Fiber A)

fibers are bonded on only one sideand recover tube-like shape more

Shape ofcross section

a

r
r

b

Assumption: fiber wall thickness is unchanged, outside and inside peripheries are ellipse-shaped.

easily. Fiber C underneath those two
fibers recovered low degree of tube-like shape. The soaking time was 3 days. So, in conclusion, kraft fibers
recover only a modest degree of tube-like shape even in the wet state.

CONCLUSIONS

Conclusions

Roughening by moisture is not a surface effect. It• Roughening by moisture is not a surface effect. It is mainly thesurface reflection of bulk changes

is mainly the surface reflection of bulk changes.

• Bulk structural changes occur as a result of water molecules
diffusing into the fiber wall leading to:– volumetric expansion in cross-section (swelling stresses)– plasticization of the hemicelluloses thereby relaxing “dried-in”
shrinkage stresses as well as the calendering-induced stresses thatkeep the fiber cross-section collapsed.
– Debonding of fibers at cross-over points, as a result of swelling andshape recovery.

Bulk structural changes occur as a result of watermolecules diffusing into the fiber wall leading to:volumetric expansion in cross-section (swelling
stresses), plasticization of the hemicelluloses
thereby relaxing “dried-in” shrinkage stresses aswell as the calendering-induced stresses that keep

the fiber cross-section collapsed. Debonding of fibers at cross-over points, as a result of swelling and shaperecovery.

REFERENCES
1) Forseth, T. and Helle, T., ”Effect of moistening on cross-sectional details of calendered paper containing
mechanical pulp”, J. Pulp and Paper Science 23(3), J95-100(1997)8

4th International Symposium -Moisture and creep effects on paper, board and containers-, EFPG Grenoble, France, 57-58(March 18-19, 1999)

2) Forsberg, P. and Lepoutre, P., “Environmental Scanning Electron Microscope examination of paper inhigh moisture environment: surface structural changes and electron beam damage”, Scanning
Microscopy 8(1), 31-34(1994)
3) Enomae, T. and Lepoutre, P., "Gloss relaxation processes: surface roughening by water", Journal of Pulp
and Paper Science, 23(7), J1-J7(1997).
4) Enomae, T. and Lepoutre, P., "Observation of the swelling behavior of kraft fibers and sheets in theenvironmental scanning electron microscope", Nordic Pulp Paper Res J, 13(4):280-284(1998).

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